FIG. 1 shows a structure of a conventional light-emitting diode. The light-emitting diode comprises a permanent substrate 109, a light-emitting stack 102, a metal reflective layer 106, a barrier layer 107, and a metal connecting structure 108 disposed on the permanent substrate 109. In addition, a first electrode 110E1 and an extending electrode 110E1′ are disposed on the light-emitting stack 102, and a second electrode 110E2 is disposed on the permanent substrate 109 for current conduction.
The metal reflective layer 106 is for reflecting light emitted by the light-emitting stack 102. The metal connecting structure 108 is for connecting the permanent substrate 109 and the barrier layer 107 by connecting two layers of materials to form an alloy. The barrier layer 107 is disposed between the metal reflective layer 106 and the metal connecting structure 108 to prevent metal diffusion between the metal reflective layer 106 and the metal connecting structure 108. However, the formation of the metal connecting structure 108 is generally performed at a high temperature, for example, a temperature higher than 300° C., and the composition of the metal connecting structure 108 are usually different from composition of the metal reflective layer 106. In other words, composition of the metal connecting structure 108 and composition of the metal reflective layer 106 do not comprise the same metal element. For example, silver (Ag) is used for the conventional metal reflective layer 106, and an alloy with zinc (Zn) as the main component, such as an alloy of zinc (Zn) and aluminum (Al), is used for the metal connecting structure 108 to facilitate a high-temperature bonding. When the metal reflective layer 106 and the metal connecting structure 108 do not comprise the same metal element, a thin barrier layer (less than 100 nm) for the conventional barrier layer 107 is able to prevent metal diffusion between the metal reflective layer 106 and the metal connecting structure 108.
However, with the development of applications of the light-emitting diode, the requirement for performance is gradually increased. For example, when a light-emitting diode is used in the automotive field, for the safety concern, requirement for the reliability for a light-emitting diode used in the automotive field is higher than that for a light-emitting diode used in the display field. Therefore, a material with better stability is required for the metal reflective layer. In addition, compared with silver which tends to have electro-migration, using other metal material as the metal reflective layer 106 has advantages. Furthermore, the low-temperature bonding of the metal connecting structure 108 becomes a trend, the material selection of the metal connecting structure 108 needs to be diversified. When the metal reflective layer 106 and the metal connecting structure 108 comprise the same metal element, because there is same metal element existing on both sides of the barrier layer 107, other elements in the alloy of the metal connecting structure 108 are particularly easy to be combined with the same metal element on both sides of the barrier layer 107. Accordingly, the design of thin barrier layer is unable to effectively prevent metal diffusion between the metal reflective layer 106 and the metal connecting structure 108. After the high-temperature processing steps in the manufacturing processes, the metal element in the metal connecting structure 108 tends to diffuse to the metal reflective layer 106 to cause a decrease in reflectivity of the metal reflective layer 106, and the luminous intensity of the light-emitting diode is decreased.